As a new lighting source, LED has many significant advantages, such as high lighting efficiency, long life. Typically, LED has two terminals of anode and cathode, as shown in FIG. 1, and is not flexible enough in application. For example, it is difficult to power LEDs in series directly by DC source: LEDs will be damaged by over current flowing through when the applied voltage is too high; or LEDs cannot be derived normally because the current flowing through is insufficient when the applied voltage is too low. When used in series, all LEDs cannot work properly only if one failed/open and results a lower reliability.